The conventional technique of attaching parts together to form large assemblies in the aerospace industry is with the use of fasteners, such as rivets, bolts, lockbolts, etc. Installation of fasteners is a laborious process, requiring the drilling holes, installing fasteners and securing the fastener, usually one at a time. The holes sometimes must be reamed or cold worked, and the interface between the parts often must be sealed as a guard against corrosion and air or fluid leaks from interior regions under pressure. A significant percentage of the cost of a final assembly, such as a wing box, that is fastened together using conventional fasteners, is in the costs of sealant application and fastener installation and the inevitable rework and scrap that results for the use of fasteners. Enormous resources have been devoted to improving the quality and reliability of fastener installation and to reducing the associated costs, but it remains a troublesome technology, long overdue for radical improvement.
Fusion bonding, or "welding" of thermoplastic composite material, such as polyimide impregnated graphite, is an emerging technology of great promise in the aerospace field for reducing the cost of fastening parts, such as wing spars, ribs and wing skins, together to make large assemblies, such as a wing box. As the use of plastic parts increases in the aerospace field and as their long-term performance becomes better understood, plastic parts will gradually replace aluminum parts and the assembly technologies will assume great economic significance in the aerospace industry. Even today, where plastic parts are widely used of fighter aircraft, fastener technologies have a substantial influence on the cost of the final product.
An apparatus and method for performing welding of thermoplastic composite material is described in U.S. patent application Ser. No. 08/352,991 filed on Dec. 9, 1994, by John Mittleider and entitled "Thermoplastic Welding", the disclosure of which is incorporated herein by reference. The method described in the Mittleider application uses a conductive foraminous susceptor at the interface between two parts to be welded together. Eddy currents in the susceptor, induced by an alternating magnetic field generated by an induction coil, heat the susceptor by resistive heating and raise the temperature of the thermoplastic in the faying surfaces of the two parts in contact with the susceptor to the thermoplastic melt point. Pressure is applied to squeeze the two parts together, pressing the melted thermoplastic through and around the interstices of the susceptor and promoting molecular diffusion of the faying surfaces to form a bond region that is continuous and uniform from one part to the other with no discernible junction between the two parts, other than the embedded susceptor.
In the Mittleider system, pressure is applied with the use of a "skate" that supports the coil and applies pressure on the parts with the use of a pressure part on each side of the coil. The skate is pulled over the parts along the band to be welded and the two pressure parts on either side of the coil press against the top part while the coil in the center heats the interface between the parts, creating a melt pool in which the thermoplastic from the faying surfaces of the two parts commingle under the pressure exerted by the pressure parts to form a bond region with no discernible junction line between the two parts. The reaction force exerted by the skate in the direction opposite to the force exerted by the pressure parts is borne by a pair of headers which guide and support the skate as it is pulled along over the parts.
Although the skate in the Mittleider system works well in the wing box assembly jig described in the aforesaid application, there may be some applications in which a simpler system would be preferable. Where the contour of the top part is more complex or the surface is textured or uneven, a tooling system that is capable of applying uniform pressure on the parts in line with the melt pool at the bond line, without sliding contact between the skate and the top part, could be advantageous.
Thus, the art of thermoplastic welding would be advanced by a tooling system for applying uniform pressure to the top part in line with the melt pool at the interface between the two parts to be fusion bonded by inductive welding using a moving coil.